Methods of attaching drug moieties to binding molecules to facilitate targeting of drug moieties to specific cells or molecules are known in the art. However, using the prior art methods limits the number of drug moieties that are attached to each binding molecule. In addition, the prior art methods are limited in that they often result in attachment of drug moieties to sites that interfere with the ability of a binding molecule to bind to its target. The development of novel methods for linking drug or labeling moieties to binding molecules would be of great benefit.